Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

ABSTRACT

A negative electrode for a nonaqueous electrolyte with a negative electrode mixture layer that includes a first layer and a second layer. The first layer is formed on the negative electrode current collector and includes a negative electrode active material and a first binding agent. The negative electrode active material in the first layer includes a carbon material A and a Si-containing compound. The first binding agent includes polyacrylic acid or a salt thereof. The second layer is formed on the first layer and includes a negative electrode active material and a second binding agent. The negative electrode active material in the second layer includes a carbon material B. The carbon material B has a tap density higher than a tap density of the carbon material A. A packing density of the second layer is lower than a packing density of the first layer.

TECHNICAL FIELD

The present invention relates to a technology for a negative electrodefor a nonaqueous electrolyte secondary battery and a technology for anonaqueous electrolyte secondary battery.

BACKGROUND ART

It is known that more lithium ions can be stored in Si-containingcompounds, such as silicon oxide represented by SiO_(x), than incarbon-based active materials, such as graphite, per unit volume.

For example, PTL 1 discloses a nonaqueous electrolyte secondary batterythat includes silicon oxide as a negative electrode active material andin which polyacrylic acid is used as a binding agent for a negativeelectrode mixture layer. Furthermore, PTL 1 also discloses that graphiteand a Si-containing compound are used in combination.

CITATION LIST Patent Literature

PTL 1: Japanese Published Unexamined Patent Application No. 2000-348730

SUMMARY OF INVENTION

In negative electrodes in which a negative electrode active materialincluding a Si-containing compound is used, one problem is thedegradation of charge-discharge cycle characteristics. A Si-containingcompound undergoes a significant volume change in association withcharging and discharging, and as a result, the degree of contact betweenthe negative electrode active material particles decreases or the stateof contact between the particles is lost. It is believed that thisresults in an increase in the number of negative electrode activematerial particles that are isolated from the conductive paths in thenegative electrode mixture layer, and, consequently, charge-dischargecycle characteristics are degraded. To inhibit an increase in the numberof such isolated negative electrode active material particles, it may beconceivable to increase the content of a binding agent. In this case,however, the resistance of the negative electrode mixture layerincreases, for example, and as a result, the input characteristics of anonaqueous electrolyte secondary battery may be degraded.

Accordingly, an object of the present disclosure is to provide anegative electrode for a nonaqueous electrolyte secondary battery, thenegative electrode being designed to inhibit degradation of thecharge-discharge cycle characteristics of a nonaqueous electrolytesecondary battery and also improve the input characteristics thereof, ina case where a Si-containing compound is included as a negativeelectrode active material.

According to an aspect of the present disclosure, a negative electrodefor a nonaqueous electrolyte secondary battery includes a negativeelectrode current collector and a negative electrode mixture layerformed on the negative electrode current collector. The negativeelectrode mixture layer includes a first layer and a second layer. Thefirst layer is formed on the negative electrode current collector andincludes a negative electrode active material and a first binding agent.The negative electrode active material in the first layer includes acarbon material A and a Si-containing compound. The first binding agentincludes polyacrylic acid or a salt thereof. The second layer is formedon the first layer and includes a negative electrode active material anda second binding agent. The negative electrode active material in thesecond layer includes a carbon material B. The carbon material B has atap density higher than a tap density of the carbon material A. A massof the first layer relative to a mass of the negative electrode mixturelayer is 50 mass % or greater and less than 90 mass %. A mass of thesecond layer relative to the mass of the negative electrode mixturelayer is greater than 10 mass % and 50 mass % or less. A packing densityof the second layer is lower than a packing density of the first layer.

According to an aspect of the present disclosure, a nonaqueouselectrolyte secondary battery includes the negative electrode for anonaqueous electrolyte secondary battery; a positive electrode; and anonaqueous electrolyte.

With an aspect of the present disclosure, degradation of thecharge-discharge cycle characteristics of a nonaqueous electrolytesecondary battery is inhibited, and also, the input characteristicsthereof are improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a nonaqueous electrolyte secondarybattery, which is an example of an embodiment.

FIG. 2 is a sectional view of a negative electrode, which is an exampleof an embodiment.

DESCRIPTION OF EMBODIMENTS

According to an aspect of the present disclosure, a negative electrodefor a nonaqueous electrolyte secondary battery includes a negativeelectrode current collector and a negative electrode mixture layerformed on the negative electrode current collector. The negativeelectrode mixture layer includes a first layer and a second layer. Thefirst layer is formed on the negative electrode current collector andincludes a negative electrode active material and a first binding agent.The negative electrode active material in the first layer includes acarbon material A and a Si-containing compound. The first binding agentincludes polyacrylic acid or a salt thereof. The second layer is formedon the first layer and includes a negative electrode active material anda second binding agent. The negative electrode active material in thesecond layer includes a carbon material B. The carbon material B has atap density higher than a tap density of the carbon material A. A massof the first layer relative to a mass of the negative electrode mixturelayer is 50 mass % or greater and less than 90 mass %. A mass of thesecond layer relative to the mass of the negative electrode mixturelayer is greater than 10 mass % and 50 mass % or less. A packing densityof the second layer is lower than a packing density of the first layer.

The carbon material B, which is included in the second layer and has ahigh tap density, is a material that enables lithium ions to beintercalated into the carbon material at a high rate and, therefore, hashigh ion acceptance with respect to, for example, lithium ions, comparedwith the carbon material A, which is included in the first layer and hasa low tap density. Furthermore, the second layer, which has a lowpacking density, has high wetting characteristics with respect toelectrolyte solutions that are nonaqueous electrolytes and, therefore,has a large number of ion conductive paths (lithium ion conductivepaths), compared with the first layer, which has a high packing density.The second layer, as described, is formed on the first layer, which isformed on the negative electrode current collector. This configurationimproves the input characteristics of a nonaqueous electrolyte secondarybattery. Furthermore, the polyacrylic acid or salt thereof included inthe first layer strongly binds particles of the negative electrodeactive material (the Si-containing compound and the carbon material A)together. Hence, even in a case where the Si-containing compoundundergoes a significant volume change in association with charging anddischarging, an increase in the number of negative electrode activematerial particles that are isolated from conductive paths in the firstlayer is inhibited. Accordingly, degradation of the charge-dischargecycle characteristics of a nonaqueous electrolyte secondary battery isinhibited.

Note that in this specification, the phrase “a value (1) to a value (2)”means a value (1) or greater and a value (2) or less.

The following describes an example of a nonaqueous electrolyte secondarybattery, in which a positive electrode active material for a nonaqueouselectrolyte secondary battery according to an aspect of the presentdisclosure is used.

FIG. 1 is a perspective view of a nonaqueous electrolyte secondarybattery, which is an example of an embodiment. A nonaqueous electrolytesecondary battery 10, illustrated in FIG. 1, is a prismatic battery;however, nonaqueous electrolyte secondary batteries of the presentdisclosure are not limited to this configuration and may be, forexample, a cylindrical battery, a laminated battery, or the like.

The nonaqueous electrolyte secondary battery 10 illustrated in FIG. 1includes an electrode assembly 11, a nonaqueous electrolyte, and abattery case 14, which houses therein the electrode assembly 11 and thenonaqueous electrolyte. The electrode assembly 11 includes a positiveelectrode, a negative electrode, and a separator. The electrode assembly11 is an electrode assembly having a stacked configuration in whichpositive and negative electrodes are alternately stacked with aseparator disposed therebetween. However, the electrode assembly is notlimited to this configuration and may be, for example, an electrodeassembly having a wound configuration in which positive and negativeelectrodes are wound together with a separator disposed therebetween.

The battery case 14 includes a case body 15, which is generallybox-shaped, a sealing member 16, which closes an opening of the casebody 15, a positive electrode terminal 12, which is electricallyconnected to the positive electrode, and a negative electrode terminal13, which is electrically connected to the negative electrode. The casebody 15 and the sealing member 16 are formed of, for example, a metalmaterial that primarily includes aluminum. The positive electrodeterminal 12 and the negative electrode terminal 13 are secured to thesealing member 16 with an insulating member 17 disposed therebetween.Note that, typically, a gas outlet feature (not illustrated) is providedin the sealing member 16. The form of the battery case 14 is not limitedto the form described above, and a form known in the art may beemployed.

The following describes in detail the positive electrode, the negativeelectrode, the nonaqueous electrolyte, and the separator that are usedin the nonaqueous electrolyte secondary battery that is an example of anembodiment.

<Positive Electrode>

The positive electrode includes a positive electrode current collectorand a positive electrode mixture layer formed on the positive electrodecurrent collector. The positive electrode current collector may be madeof a metal that is stable in the potential range of the positiveelectrode. The metal may be aluminum, for example, and may be in theform of a foil, a film in which the metal is disposed in a surfacelayer, or the like. The positive electrode mixture layer includes, forexample, a positive electrode active material, a binding agent, aconductive material, and the like. The positive electrode mixture layeris formed, for example, on both sides of the positive electrode currentcollector. The positive electrode may be obtained in the followingmanner, for example. A positive electrode mixture slurry including apositive electrode active material, a binding agent, a conductivematerial, and the like is applied onto the positive electrode currentcollector, and then the applied layer is dried to form a positiveelectrode active material layer on the positive electrode currentcollector. The positive electrode active material layer is then rolled.

The positive electrode active material includes a lithium transitionmetal oxide, for example. A metal element included in the lithiumtransition metal oxide is at least one selected from, for example,magnesium (Mg), aluminum (Al), calcium (Ca), scandium (Sc), titanium(Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt(Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge),yttrium (Y), zirconium (Zr), tin (Sn), antimony (Sb), tungsten (W), lead(Pb), and bismuth (Bi). Among these, it is preferable to include atleast one selected from Co, Ni, Mn, and Al.

Examples of the conductive material included in the positive electrodemixture layer include carbon materials, such as carbon black (CB),acetylene black (AB), Ketjen black, and graphite. These may be usedalone or in a combination of two or more.

Examples of the binding agent included in the positive electrode mixturelayer include fluororesins, such as polytetrafluoroethylene (PTFE) andpolyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-basedresins, acrylic-based resins, and polyolefin-based resins. These may beused alone or in a combination of two or more.

<Negative Electrode>

FIG. 2 is a sectional view of a negative electrode, which is an exampleof an embodiment. As illustrated in FIG. 2, a negative electrode 20includes a negative electrode current collector 30 and a negativeelectrode mixture layer 31, which is formed on the negative electrodecurrent collector 30. The negative electrode current collector 30 may bemade of a metal that is stable in the potential range of the negativeelectrode. The metal may be copper, for example, and may be in the formof a foil, a film in which the metal is disposed in a surface layer, orthe like. The negative electrode mixture layer 31 is formed, forexample, on both sides of the negative electrode current collector 30.

The negative electrode mixture layer 31 has a two-layer structure thatincludes a first layer 32, which is formed on the negative electrodecurrent collector 30, and a second layer 33, which is formed on thefirst layer 32. The first layer 32 includes a negative electrode activematerial and a first binding agent. The negative electrode activematerial in the first layer includes a carbon material A and aSi-containing compound. The first binding agent includes polyacrylicacid (PAA) or a salt thereof. The second layer 33 includes a negativeelectrode active material and a second binding agent. The negativeelectrode active material in the second layer includes a carbon materialB.

A mass of the first layer 32 relative to a mass of the negativeelectrode mixture layer 31 is 50 mass % or greater and less than 90 mass% and is preferably 50 mass % to 70 mass %. A mass of the second layer33 relative to the mass of the negative electrode mixture layer 31 isgreater than 10 mass % and 50 mass % or less and is preferably 30 mass %to 50 mass %. When the mass of the first layer 32 relative to the massof negative electrode mixture layer 31 is 90 mass % or greater, and themass of the second layer 33 relative to the mass of the negativeelectrode mixture layer 31 is 10 mass % or less, the proportion of thesecond layer 33, which contributes to input characteristics, is small.Thus, the input characteristics of a nonaqueous electrolyte secondarybattery are degraded compared with cases other than the cases of theabove-mentioned ranges. Furthermore, when the mass of the first layer 32relative to the mass of the negative electrode mixture layer 31 is lessthan 50 mass %, and the mass of the second layer 33 relative to the massof the negative electrode mixture layer 31 is greater than 50 mass %,the proportion of the first layer 32 is small (that is, the amount ofthe Si-containing compound is reduced). Thus, it is difficult to achievean increase in battery capacity compared with cases other than the casesof the above-mentioned ranges.

It is preferable that a thickness of the negative electrode mixturelayer 31 be, for example, 30 μm to 100 μm, on one side of the negativeelectrode current collector 30. More preferably, the thickness is 50 μmto 80 μm.

It is preferable that a packing density of the negative electrodemixture layer 31 be greater than or equal to 1.65 g/cm³, from thestandpoint of, for example, improving battery capacity. More preferably,the packing density is within a range of 1.65 g/cm³ to 1.75 g/cm³.

It is sufficient that a packing density of the second layer 33 be lowerthan a packing density of the first layer 32, in terms of improving theinput characteristics of a nonaqueous electrolyte secondary battery. Itis preferable, however, that the packing density of the second layer 33be within a range of, for example, 1.40 to 1.55 g/cm³. It is preferablethat the packing density of the first layer 32 be within a range of, forexample, 1.70 to 1.95 g/cm³, from the standpoint of improving thecapacity of a nonaqueous electrolyte secondary battery and inhibitingdegradation of the cycle characteristics thereof.

Examples of the carbon material A included in the first layer 32 and thecarbon material B included in the second layer 33 include graphite andamorphous carbon. Among these, graphite is preferable. Examples of thegraphite include natural graphite, such as flake graphite, andartificial graphite, such as massive artificial graphite (MAG) andgraphitized mesophase carbon microbeads (MCMB). Furthermore, thegraphite may be a composite in which, for example, graphite is coatedwith amorphous carbon.

The carbon material A included in the first layer 32 and the carbonmaterial B included in the second layer 33 may be an identical materialor different materials provided that the tap density of the carbonmaterial B is greater than the tap density of the carbon material A, aswill be described later. It is preferable that the carbon material Aincluded in the first layer 32 be a carbon material that can mitigatevolume changes in the Si-containing compound. It is preferable that thesecond layer 33 be a carbon material that has increased ion acceptancewith respect to, for example, lithium ions; such a carbon material maybe obtained by, for example, forming a composite with amorphous carbonthat has a high lithium ion intercalation and deintercalation rate. Thecarbon material A and the carbon material B may each be one materialused alone or two or more materials used in combination. For example,two or more materials, such as flake graphite and massive artificialgraphite, may be used in combination as the carbon material A, and onematerial, such as graphite with surfaces of the particles coated withamorphous carbon, may be used alone as the carbon material B.

The tap density of the carbon material B included in the second layer 33is higher than the tap density of the carbon material A included in thefirst layer 32. This is to improve the input characteristics of anonaqueous electrolyte secondary battery. For example, the tap densityof the carbon material B is preferably greater than or equal to 1.00g/cm³ and preferably within a range of 1.00 g/cm³ to 1.25 g/cm³.Furthermore, a material of the carbon material B is preferably graphite.When the tap density of the carbon material B included in the secondlayer 33 is within any of the above-mentioned ranges, ion acceptancewith respect to, for example, lithium ions may be enhanced, which mayresult in further improved input characteristics of a nonaqueouselectrolyte secondary battery, compared with cases other than the casesof the above-mentioned ranges. It is sufficient that the tap density ofthe carbon material A included in the first layer 32 be lower than thetap density of the carbon material B included in the second layer 33.However, for example, the tap density of the carbon material A ispreferably within a range of 0.85 g/cm³ to 1.00 g/cm³ and morepreferably within a range of 0.89 to 0.95. Furthermore, a material ofthe carbon material A is preferably graphite. When the tap density ofthe carbon material B included in the first layer 32 is within any ofthe above-mentioned ranges, volume changes in the Si-containing compoundmay be mitigated, which may result in further inhibited degradation ofthe charge-discharge cycle characteristics of a nonaqueous electrolytesecondary battery, compared with cases other than the cases of theabove-mentioned ranges. The tap density of the carbon materials isdetermined as a bulk density obtained after sample powder contained in avessel was tapped 250 times, in accordance with the method specified inJIS Z-2504.

It is preferable that a BET specific surface area of the carbon materialB included in the second layer 33 be higher than a BET specific surfacearea of the carbon material A included in the first layer 32, from thestandpoint of, for example, improving the input characteristics of anonaqueous electrolyte secondary battery. For example, the BET specificsurface area of the carbon material B is preferably within a range of4.0 m²/g to 8.0 m²/g and more preferably within a range of 4.2 to 7.0m²/g. When the BET specific surface area of the carbon material Bincluded in the second layer 33 is within any of the above-mentionedranges, ion acceptance with respect to, for example, lithium ions may beenhanced, which may result in further improved input characteristics ofa nonaqueous electrolyte secondary battery, compared with cases otherthan the cases of the above-mentioned ranges. The BET specific surfacearea of the carbon material A included in the first layer 32 ispreferably lower than the BET specific surface area of the carbonmaterial B included in the second layer 33 and is preferably, forexample, within a range of 0.9 m²/g to 4.5 m²/g. When the BET specificsurface area of the carbon material A included in the first layer 32 iswithin the above-mentioned range, decreases in charge-dischargeefficiency and capacity retention rate due to a side reaction with theelectrolyte solution during charging and discharging may be likelyinhibited compared with cases other than the cases of theabove-mentioned range. The BET specific surface area is measured inaccordance with the BET method (nitrogen adsorption method) described inJIS R1626.

Typically, the carbon material A included in the first layer 32 and thecarbon material B included in the second layer 33 are secondaryparticles, that is, aggregates of multiple primary particles. Theaverage particle diameter of secondary particles of the carbon materialA and the carbon material B is not particularly limited and is, forexample, 1 μm to 30 μm. The average particle diameter of secondaryparticles is a volume average particle diameter (Dv50), whichcorresponds to a volume integrated value that equals 50% in a particlesize distribution measured using the laser diffraction light scatteringmethod.

The Si-containing compound included in the first layer 32 is notparticularly limited provided that the compound contains Si. Preferably,the Si-containing compound is silicon oxide represented by SiO_(x)(0.5×1.5). One Si-containing compound may be used alone, or two or moreSi-containing compounds may be used in combination. It is preferablethat the particles of the silicon oxide, such as SiO_(x), have, onsurfaces, a conductive coating including a material having a higherconductivity than the silicon oxide. The average particle diameter(Dv50) of the silicon oxide, such as SiO_(x), is, for example, within arange of 1 μm to 15 μm.

For example, the SiO, has a structure in which Si is dispersed in anamorphous SiO₂ matrix. Furthermore, the SiO_(x) may contain lithiumsilicate (e.g., lithium silicate represented by Li_(2z)SiO_((2+z))(0<z<2)) in the particles or may have a structure in which Si isdispersed in a lithium silicate phase.

It is preferable that the conductive coating be a carbon coating. Thecarbon coating is in an amount within a range of, for example, 0.5 mass% to 10 mass % relative to a mass of the SiO_(x) particles. Examples ofmethods for forming the carbon coating include the following: a methodin which coal tar or the like is mixed with SiO_(x) particles, and themixture is heat-treated; and a chemical vapor deposition method (CVDmethod) using a hydrocarbon gas or the like. Alternatively, the carboncoating may be formed by adhering a binder, such as carbon black orKetjen black, to surfaces of the SiO_(x) particles.

It is preferable that a mass ratio between the carbon material A and theSi-containing compound, which are included in the first layer 32, bewithin a range of, for example, 95:5 to 70:30, from the standpoint of,for example, mitigating, via the carbon material A, volume changes inthe Si-containing compound to inhibit degradation of thecharge-discharge cycle characteristics of a nonaqueous electrolytesecondary battery. More preferably, the range is 95:5 to 80:20.

It is sufficient that the first binding agent included in the firstlayer 32 contain PAA or a salt thereof (e.g., a salt, such as a lithiumsalt, a sodium salt, a potassium salt, or an ammonium salt, a partiallyneutralized salt, or the like). It is preferable, however, that anadditional binding agent be included. Examples of the additional bindingagent include carboxymethyl cellulose (CMC) and salts thereof,styrene-butadiene copolymers (SBR), polyvinyl alcohol (PVA),polyethylene oxide (PEO), and derivatives of the foregoing compounds.

Examples of the second binding agent included in the second layer 33include CMC and salts thereof, SBR, PVA, PEO, and derivatives of theforegoing compounds. The second layer 33 may contain PAA or a saltthereof. It is preferable, however, that the second layer 33 containsubstantially no PAA or salt thereof, from the standpoint of improvingthe input characteristics of a nonaqueous electrolyte secondary battery.

With regard to contents of the first binding agent included in the firstlayer 32 and the second binding agent included in the second layer 33,it is preferable that the content of the first binding agent included inthe first layer 32 be higher than the content of the second bindingagent included in the second layer 33, from the standpoint of, forexample, inhibiting degradation of the charge-discharge cyclecharacteristics of a nonaqueous electrolyte secondary battery andimproving the input characteristics of a nonaqueous electrolytesecondary battery.

The content of the first binding agent included in the first layer 32 ispreferably, for example, within a range of 0.5 mass % to 10 mass % andmore preferably within a range of 1 mass % to 5 mass %, relative to amass of the first layer 32. The content of the PAA or salt thereof inthe first binding agent is preferably, for example, greater than orequal to 20 mass % and more preferably greater than or equal to 30 mass%, relative to a mass of the first binding agent. When the content ofthe PAA or salt thereof in the first binding agent is greater than orequal to 20 mass %, the amount of the negative electrode active materialthat is isolated from the conductive paths in the first layer 32 may bereduced, and thus degradation of charge-discharge cycle characteristicsmay be more likely inhibited than in cases in which the content is lessthan 20 mass %.

The content of the second binding agent included in the second layer 33is preferably, for example, within a range of 0.5 mass % to 10 mass %and more preferably within a range of 1 mass % to 5 mass %, relative toa mass of the second layer 33. Note that it is preferable that thesecond binding agent contain substantially no PAA or salt thereof asmentioned above. For example, it is preferable that PAA or a saltthereof be present in an amount less than 0.1 mass % relative to themass of the second binding agent.

The negative electrode active material included in the second layer 33may contain a Si-containing compound as mentioned above in addition tothe carbon material B mentioned above. However, from the standpoint ofimproving the input characteristics of a nonaqueous electrolytesecondary battery, it is preferable that the negative electrode activematerial contain the carbon material B exclusively and containsubstantially no Si-containing compound. For example, it is preferablethat a content of the Si-containing compound in the second layer 33 beless than 0.1 mass % relative to the mass of the second layer 33.

The negative electrode 20 is produced in the following manner, forexample. A first negative electrode mixture slurry for the first layer,which includes a negative electrode active material, a first bindingagent, and the like, is prepared. The negative electrode active materialcontains a carbon material A and a Si-containing compound. The firstbinding agent contains PAA or a salt thereof. In addition, a secondnegative electrode mixture slurry for the second layer, which includes anegative electrode active material, a second binding agent, and thelike, is prepared. The negative electrode active material contains acarbon material B. Subsequently, the first negative electrode mixtureslurry is applied onto the negative electrode current collector 30, andthen the applied layer is dried. Accordingly, the first layer 32 isformed on the negative electrode current collector 30. Next, the secondnegative electrode mixture slurry is applied onto the first layer 32,which has been formed on the negative electrode current collector 30,and then the applied layer is dried. Accordingly, the second layer 33 isformed on the first layer 32. The first layer 32 and the second layer 33are then rolled. In this manner, the negative electrode 20 can beobtained. In the negative electrode 20, the negative electrode mixturelayer 31, which includes the first layer 32 and the second layer 33, isformed on the negative electrode current collector 30.

<Nonaqueous Electrolyte>

The nonaqueous electrolyte includes a nonaqueous solvent and anelectrolyte salt dissolved in the nonaqueous solvent. The nonaqueouselectrolyte is not limited to a liquid electrolyte (nonaqueouselectrolyte solution) and may be a solid electrolyte including a gelpolymer or the like. Examples of the nonaqueous solvent include esters,such as ethylene carbonate (EC), propylene carbonate (PC), dimethylcarbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC),and methyl propionate (MP); ethers; nitriles; amides; and mixed solventsof two or more of the foregoing compounds. The nonaqueous solvent maycontain a halogen-substituted compound that corresponds to any of theabove-mentioned solvents and in which at least one hydrogen atom isreplaced with a halogen atom, such as a fluorine atom. Examples of thehalogen-substituted compound include fluorinated cyclic carbonic acidesters, such as fluoroethylene carbonate (FEC); fluorinated chaincarbonate esters; and fluorinated chain carboxylic acid esters, such asfluoromethyl propionate (FMP). It is preferable that the nonaqueouselectrolyte contain fluoroethylene carbonate present in an amountgreater than or equal to 15 mass %, from the standpoint of, for example,inhibiting degradation of the charge-discharge cycle characteristics ofa nonaqueous electrolyte secondary battery or improving the inputcharacteristics thereof. More preferably, the nonaqueous electrolytecontains fluoroethylene carbonate present within a range of 15 mass % to25 mass %. Examples of the electrolyte salt include lithium salts, suchas LiBF₄ and LiPF₆.

<Separator>

For the separator, a porous sheet having ion permeability and insulatingproperties is used, for example. Specific examples of the porous sheetinclude microporous membranes, woven fabrics, and nonwoven fabrics.Suitable materials for the separator include olefin-based resins, suchas polyethylene, polypropylene, and copolymers including at least one ofethylene and propylene, and cellulose. The separator may be a layeredstructure including a cellulose fiber layer and a thermoplastic resinfiber layer. The thermoplastic resin may be an olefin-based resin, forexample. Furthermore, the separator may be a multi-layer separatorincluding a polyethylene layer and a polypropylene layer, for example.Furthermore, the separator may include an aramid-based resin or the likeapplied onto a surface thereof. Furthermore, a heat resistant layerincluding an inorganic filler may be formed on at least one of theinterface between the separator and the positive electrode and theinterface between the separator and the negative electrode.

EXAMPLES

The present invention will be further described below with reference toexamples, but the present invention is not limited to the examples.

Example 1

[Positive Electrode]

94.8 parts by mass of a lithium transition metal oxide represented byLiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, which was used as a positive electrodeactive material, 4 parts by mass of acetylene black (AB), and 1.2 partsby mass of polyvinylidene fluoride (PVDF) were mixed together, and thenan appropriate amount of N-methyl-2-pyrrolidone (NMP) was added. Thus, apositive electrode mixture slurry was adjusted. Next, the positiveelectrode mixture slurry was applied to a positive electrode currentcollector, which was made of aluminum foil, except for a portion towhich a lead was to be connected, and then the applied layer was dried.The applied layer was rolled by using a roller, and thereafter theresultant was cut to a predetermined electrode size. Thus, a positiveelectrode including positive electrode mixture layers formed on bothsides of the positive electrode current collector was produced.

[Negative Electrode]

89 parts by mass of graphite A, 8 parts by mass of SiO_(x) (x=0.94)having a carbon coating, 1 part by mass of a lithium salt of PAA, 1 partby mass of a sodium salt of CMC, and 1 part by mass of SBR were mixedtogether, and then an appropriate amount of water was added. Thegraphite A had a tap density of 0.92 g/cm³ and a BET specific surfacearea of 4.2 m²/g and was used as a carbon material A. Thus, a firstnegative electrode mixture slurry for the first layer was adjusted.Furthermore, 97.5 parts by mass of graphite B1, 1.5 parts by mass of asodium salt of CMC, and 1 part by mass of SBR were mixed together, andthen an appropriate amount of water was added. The graphite B1 had a tapdensity of 1.14 g/cm³ and a BET specific surface area of 6.1 m²/g andwas used as a carbon material B. Thus, a second negative electrodemixture slurry for the second layer was prepared.

Next, the first negative electrode mixture slurry was applied to bothsides of a negative electrode current collector, which was made ofcopper foil, except for a portion to which a lead was to be connected,and then the applied layers were dried. Thus, first layers were formedon both sides of the negative electrode current collector. Next, thesecond negative electrode mixture slurry was applied to the first layersformed on both sides of the negative electrode current collector, andthen the applied layers were dried. Thus, second layers were formed.Subsequently, the applied layers were rolled by using a roller, andthereafter the resultant was cut to a predetermined electrode size.Thus, a negative electrode including negative electrode mixture layersformed on both sides of the negative electrode current collector wasproduced. Each of the negative electrode mixture layers included firstand second layers. Masses and thicknesses of the first layers and thesecond layers of the negative electrode mixture layers were measured.The results were that the mass ratio of the second layer to the firstlayer was 1.0, and the thickness ratio of the second layer to the firstlayer was 1.34. That is, the mass of the first layers relative to themass of the negative electrode mixture layers was 50 mass %, the mass ofthe second layers relative to the mass of the negative electrode mixturelayers was 50 mass %, and the second layer had a lower packing densitythan the first layer. Note that the packing density of the negativeelectrode mixture layers was 1.65 g/cm³.

[Nonaqueous Electrolyte]

Lithium hexafluorophosphate (LiPF₆) was added to a mixed solvent to aconcentration of 1.0 mol/L. In the mixed solvent, ethylene carbonate(EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of3:7. Furthermore, 2 vol % (relative to the solvent) of vinylenecarbonate was added. Thus, a nonaqueous electrolyte was prepared.

[Test Cell]

A lead was attached to the negative electrode and the positiveelectrode, and then the electrodes, with a separator disposedtherebetween, were wound spirally. Thus, an electrode assembly having awound structure was produced. The separator used was a single-layerseparator made of polypropylene. The produced electrode assembly wasinserted into a housing formed of an aluminum laminate sheet and wasdried under vacuum at 105° C. for 2 hours and 30 minutes. Subsequently,the nonaqueous electrolyte was introduced into the housing, and then theopening of the housing was sealed. Thus, a test cell (laminate cell) wasproduced. The design capacity of the test cell was 880 mAh.

Example 2

A negative electrode was produced as in Example 1 except that a graphiteB2 was used as a carbon material B in the preparation of the secondnegative electrode mixture slurry, and the amounts of application andthicknesses of the first layer and the second layer were changed. Thegraphite B2 had a tap density of 1.06 g/cm³ and a BET specific surfacearea of 4.4 m²/g. In the negative electrode mixture layer, the massratio of the second layer to the first layer was 0.5, and the thicknessratio of the second layer to the first layer was 0.62. That is, the massof the first layers relative to the mass of the negative electrodemixture layers was 67 mass %, the mass of the second layers relative tothe mass of the negative electrode mixture layers was 33 mass %, and thesecond layer had a lower packing density than the first layer. Note thatthe packing density of the negative electrode mixture layers was 1.70g/cm³. A test cell was produced as in Example 1 except that the negativeelectrode just described was used.

Comparative Example

A negative electrode was produced as in Example 1 except that theproduction of the negative electrode was carried out as follows. 93parts by mass of graphite A, 4 parts by mass of SiO_(x) (x=0.94) havinga carbon coating, 1 part by mass of a lithium salt of PAA, 1 part bymass of a sodium salt of CMC, and 1 part by mass of SBR were mixedtogether, and then an appropriate amount of water was added. Negativeelectrode mixture layers having a single-layer structure were formedfrom this negative electrode mixture slurry on both sides of a negativeelectrode current collector. Furthermore, a test cell was produced as inExample 1 except that the negative electrode just described was used.

[Evaluation of Capacity Retention Rate in Charge-Discharge Cycle]

In an environment at a temperature of 25° C., constant-current chargingwas performed at a constant current of 0.5 It until the cell voltagereached 4.2 V, and thereafter constant-voltage charging was performed at4.2 V until the current value reached 1/50 It. Subsequently,constant-current discharging was performed at a constant current of 0.5It until the cell voltage reached 2.5 V. This charge-discharge cycle wasperformed 50 times, and a capacity retention rate in thecharge-discharge cycle was determined in accordance with the followingequation. The results are shown in Table 1. In Table 1, the capacityretention rate of the test cell of Comparative Example 1 is thereference (1.0), and the capacity retention rates of Examples andComparative Examples are values relative to the reference.

Capacity retention rate=(X2/X1)×100

X1: discharge capacity in 1st cycle

X2: discharge capacity in 50th cycle

[Evaluation of Input Characteristics]

In an environment at a temperature of 25° C., charging was performed ata constant current of 0.5 It until half of the initial capacity wasreached. The charging was then stopped, and the cell was left to standfor 15 minutes. Subsequently, charging was performed at a current valueof 0.1 It for 10 seconds, and thereafter the voltage was measured.Subsequently, discharging corresponding to the 10-second chargedcapacity was performed. At the next current value, charging wasperformed for 10 seconds, and thereafter the voltage was measured.Subsequently, discharging corresponding to the 10-second chargedcapacity was performed. This operation was repeated from a current valueof 0.1 It to a current value of 2 It. From the measured voltage values,the current value at which 4.2 V was achieved with 10-second chargingwas calculated. Accordingly, a necessary power (input characteristics)of the instance was determined. The results are shown in Table 1. InTable 1, the necessary power of the test cell of Comparative Example 1is the reference (1.0), and the necessary powers of Examples andComparative Examples are values relative to the reference. Valuesgreater than 1.0 indicate improved input characteristics.

TABLE 1 Negative Weight ratio Thickness Necessary electrode of secondratio of power Capacity mixture layer to second layer (input retentionlayer first layer to first layer characteristics) rate Example 1 Secondlayer: graphite 1.00 1.34 1.12 1.00 B1/CMC/SBR First layer: graphiteA/SiO_(x)/PAA/CMC/SBR Example 2 Second layer: graphite 0.50 0.62 1.111.00 B2/CMC/SBR First layer: graphite A/SiO_(x)/PAA/CMC/SBR ComparativeSingle layer: graphite — — 1.00 1.00 Example 1 A/SiO_(x)/PAA/CMC/SBR

Each of the negative electrode mixture layers of Examples 1 and 2included a first layer and a second layer. The first layer was formed ona negative electrode current collector and included a negative electrodeactive material and a first binding agent. The negative electrode activematerial in the first layer included a carbon material A and aSi-containing compound. The first binding agent included polyacrylicacid or a salt thereof. The second layer was formed on the first layerand included a negative electrode active material and a second bindingagent. The negative electrode active material in the second layerincluded a carbon material B. The carbon material B had a tap densityhigher than a tap density of the carbon material A. A mass of the firstlayer relative to a mass of the negative electrode mixture layer was 50mass % or greater and less than 90 mass %. A mass of the second layerrelative to the mass of the negative electrode mixture layer was greaterthan 10 mass % and 50 mass % or less. A packing density of the secondlayer was lower than a packing density of the first layer. The negativeelectrode mixture layer of Comparative Example 1 had a single-layerstructure that included a carbon material, a Si-containing compound, anda salt of polyacrylic acid. In a comparison of Examples 1 and 2 withComparative Example 1, both Examples 1 and 2 demonstrated a highernecessary power than Comparative Example 1 and, therefore, had improvedinput characteristics. Furthermore, both Examples 1 and 2 demonstrated acapacity retention rate comparable to the capacity retention rate ofComparative Example 1, which demonstrated that degradation ofcharge-discharge cycle characteristics was inhibited.

REFERENCE SIGNS LIST

10 Nonaqueous electrolyte secondary battery

11 Electrode assembly

12 Positive electrode terminal

13 Negative electrode terminal

14 Battery case

15 Case body

16 Sealing member

17 Insulating member

20 Negative electrode

30 Negative electrode current collector

31 Negative electrode mixture layer

32 First layer

33 Second layer

1. A negative electrode for a nonaqueous electrolyte secondary battery,the negative electrode comprising a negative electrode current collectorand a negative electrode mixture layer formed on the negative electrodecurrent collector, the negative electrode mixture layer including afirst layer formed on the negative electrode current collector andincluding a negative electrode active material and a first bindingagent, the negative electrode active material, which is in the firstlayer, including a carbon material A and a Si-containing compound, thefirst binding agent including polyacrylic acid or a salt thereof, and asecond layer formed on the first layer and including a negativeelectrode active material and a second binding agent, the negativeelectrode active material, which is in the second layer, including acarbon material B, the carbon material B having a tap density higherthan a tap density of the carbon material A, wherein a mass of the firstlayer relative to a mass of the negative electrode mixture layer is 50mass % or greater and less than 90 mass %, and a mass of the secondlayer relative to the mass of the negative electrode mixture layer isgreater than 10 mass % and 50 mass % or less, and a packing density ofthe second layer is lower than a packing density of the first layer. 2.The negative electrode for a nonaqueous electrolyte secondary batteryaccording to claim 1, wherein the carbon material B has a BET specificsurface area higher than a BET specific surface area of the carbonmaterial A.
 3. The negative electrode for a nonaqueous electrolytesecondary battery according to claim 1, wherein a BET specific surfacearea of the carbon material A is within a range of 0.9 m²/g to 4.5 m²/g,and a BET specific surface area of the carbon material B is within arange of 4.0 m²/g to 8.0 m²/g.
 4. The negative electrode for anonaqueous electrolyte secondary battery according to claim 1, whereinthe tap density of the carbon material A is within a range of 0.85 g/cm³to 1.00 g/cm³.
 5. The negative electrode for a nonaqueous electrolytesecondary battery according to claim 1, wherein the tap density of thecarbon material B is within a range of 1.00 g/cm³ to 1.25 g/cm³.
 6. Thenegative electrode for a nonaqueous electrolyte secondary batteryaccording to claim 1, wherein a packing density of the negativeelectrode mixture layer is greater than or equal to 1.65 g/cm³.
 7. Anonaqueous electrolyte secondary battery comprising: the negativeelectrode for a nonaqueous electrolyte secondary battery according toclaim 1; a positive electrode; and a nonaqueous electrolyte.
 8. Thenonaqueous electrolyte secondary battery according to claim 7, whereinfluoroethylene carbonate is present in an amount greater than or equalto 15 mass % in the nonaqueous electrolyte.